Drop expired subkey from description of BF53AD52308AD4EC (Sakari Ailus)

Sakari's signing subkey expired in October last year. Drop it from the
descriptive header in the key file.

Link: https://patch.msgid.link/20260129092838.941498-2-u.kleine-koenig@baylibre.com
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>